If you'd like to use your (or one of your SIMs) SIM for long duration calls to the US and other countries I can recommend you
CallYa OpenEnd Talk&SMS ? Prepaid-Wiki
This is a Vodafone CallYa Prepaid tariff called CallYa Open End Talk&SMS which will be charged with 29€ct
per call to german landline.
The CallYa SIMs are very cheap now as they usually come with a simlocked mobile. You may find even offers for 1€ with 10€ balance.
With this method you call
DialNow | Cheap International Calls and any further minute to the US will cost you an additional €0,5ct.
For data (9€ct/min) this card is not as cheap as others (i.e.blau.de) which are 24€ct/MB
Detailed info on German prepaid offers you'll find on
Prepaid-Wiki
For international use I like my
airBalticcard | About airBalticcard Mobile (no data!).